Semiconductors are materials that have electronic properties between electrical insulators and electrical conductors. The efficiency of a semiconducting material is determined by how easily the electrons and electron ‘holes’ can move through the material—i.e. the electron and hole mobilities (μe or μh). Highly conjugated organic compounds have overlapping atomic orbitals that form valence and conducting bands similar to metals. Organic semiconductors do not have the same electron or hole mobilities as single-crystalline silicon, but they are advantageous during fabrication as solution processing techniques such as lithography can be used.
Silicon and gallium arsenide semiconductors, silicon dioxide insulators, and metals such as aluminum and copper have dominated the semiconductor industry for many years. More recently, however, organic thin-film transistors (OTFTs) have presented an alternative to the traditional thin-film transistors based on inorganic materials. For example, research efforts have focused on linear acenes (including tetracene and pentacene), thiophene oligomers (including α-sexithiophene), regioregular polythiophenes, copper phthalocyanines and naphthalenebisimides as candidates for organic semiconductors (Katz H. E. et al. Acc Chem Res (2001), 34, 359). Of these, pentacene exhibits the best electron and hole mobilities. Charge-carrier mobility values of 1.5 cm2V−1s−1, on/off current ratios greater than 108, and sub-threshold voltages of less than 1.6 V have been reported for pentacene-based transistors. Therefore, the charge-carrier mobility values for pentacenes are comparable or even superior to those of amorphous silicon-based devices.
A rapid two-step synthesis for pentacene was reported in 1972, as shown in Scheme 1, and pentacene was found to be both light and air sensitive (Goodings E. P. et al. J Chem Soc, Perkin I (1972), 1310). However, more problematic is the virtual insolubility of pentacene in common organic solvents, thereby preventing solution-based processing (Mayer zu Heingdorf F.-J. et al. Nature (2001) 412, 517). As a result, pentacene must generally be deposited from the vapor phase by vacuum sublimation in order to achieve maximum performance. The vacuum sublimation method, however, requires expensive equipment and lengthy pump-down cycles.
Scheme 1: Synthesis of Pentacene
Another disadvantage of pentacene relates to its polymorphic nature, which can have a detrimental influence upon the performance and reproducibility of pentacene-based devices. The alignment or structural order of the pentacene molecules differs for each polymorph or crystallographic phase, and this structural order determines the electronic properties of the device. The crystallographic phase adopted by pentacene depends on the method and conditions under which the crystals are formed. For example, when pentacene is vapor-deposited onto a substrate, a thin film phase is formed. This thin film phase is more effective at transporting charge than pentacene's bulk or single crystal phase, but it is meta-stable. For example, the thin film form of pentacene can be converted to the bulk phase by exposure to solvents such as isopropanol, acetone or ethanol.
More recently, substituted pentacene compounds have been developed that are more soluble in organic solvents, exhibit regular crystal packing, and are better suited for organic processing. For example, corresponding international patent publications WO03/028125, and WO03/027050, both published Apr. 3, 2003 and which are incorporated herein by reference, disclose substituted pentacene compounds and methods for their preparation. The substitutions include electron-donating groups and halogen atoms. Such petancene compounds are, at least in preferred embodiments, suited for use in organic semiconductor materials. Particularly useful semiconductor compounds include 2,9- and 2,10-disubstituted pentacenes, which are predicted to exhibit excellent solubility, solid-state packing and Σ-orbital overlap (Anthony, J. E. et al. J Am Chem Soc (2001), 123, 9482; Anthony J. E. et al. Org Lett (2002) 4, 15).
To date, the production of 2,9- and 2,10-disubstituted pentacenes has been difficult to achieve. International patent publication WO03/027050 discloses a method for preparing pentacene derivatives comprising the step of cyclizing at least one substituted bis(benzyl)phthalic acid to form the corresponding substituted pentacenedione by using an acid composition comprising trifluoromethanesulphonic acid, wherein the bis(benzyl)phthalic acid is selected from:
each R representing an electron-donating group, a halogen atom, or a hydrogen atom. In preferred embodiments, the method is suitable for generating a 2,9- or 2,10-disubstituted pentacene 5,7 or 5,12-dione, which can undergo reduction and dehydration to generate the corresponding disubstituted pentacene.
There remains a continuing need to develop novel pathways for the production of compounds comprising a linear series of five fused carbon rings, such as for example 2,9- and 2,10-disubstituted pentacene compounds, and corresponding pentacene derivatives. Moreover, there remains a need to develop methods that are better suited for large-scale production of a broad range of pentacene derivatives, and other compounds comprising a linear series of five fused carbon rings, within minimal cost. New pathways are desired to present opportunities to develop new classes of pentacene derivatives, for example with alternative substitutions either on the A and E rings, or the other rings of the five fused carbon ring core structure.